Many types of skeletal implants and skeletal joint prosthesis are available for substitutes of natural skeletal components and joints. Such prostheses are used when the natural skeletal component has been damaged by disease or trauma. However, a problem exists in the case of juvenile patient's or other patients where natural growth would otherwise be occurring. As the patient grows, the limb having the prosthesis implanted will not grow at the same rate as the opposing limb unless the prosthesis allows for growth along its length.
The U.S. Pat. No. 4,892,546 to Kotz et al, issued Jan. 9, 1990, discloses an adjustable prosthesis for a bone joint including a joint component, an elongated rod having a distal end and a proximal end, a drive trunion connected to the rod through an annular gear, an elongated inner sleeve and an outer sleeve, and means for preventing rotation of the inner sleeve within the outer sleeve. The elongated rod includes a threaded spindle on the inner sleeve and a spindle nut which cooperates with the threaded spindle to provide extension and retraction adjustment of the rod relative to the inner sleeve.
The European Patent Application EP 86109228, assigned to Waldemar Link GmbH, discloses an endoprosthesis replacing bone middle sections including a tubular spacer with a collar at each end in a solid bone nail extending from one collar. A sliding nail fits in a bore of the tubular spacer and can be locked in any required position by a lock screw.
A joint implant is disclosed in the U.S. Pat. No. 4,502,160 to Moore et al., issued Mar. 5, 1985, and assigned to the assignee of the present invention, the disclosure of which is hereby incorporated by reference and relied upon. The Moore et al. patent discloses an implant including a stem for bone implantation including a threaded portion for adjusting axially with respect to an overlying sleeve which carries an articulating component of the joint. A pin in the sleeve and an elongated slot in the stem restrain relative rotation to allow axial relative movement as the stem is urged into or out of the sleeve by rotation of a nut coacting with the threads of the stem while being axially restrained by the sleeve. The nut acts as a ring gear.
Although the Moore et al. patent discloses a very useful embodiment for a prosthetic implant, it is possible that galling of the threads of the stem may occur caused by the significant structural forces placed on the threaded members during excessive physical activity of the patient. The possibility for galling of the threads is aggravated by the relatively soft biocompatible materials required in prosthetic implant appliances.
Catalog No. 86-1 entitled "Spiralock.TM.", distributed by Detroit Tool Industries, 25219 Dequindre Road, P.O. Box 71629, Madison Heights, Mich. 48071-0629, discloses a thread forming female tap which produces a wedge ramp along the root portion of the female thread form. The entire disclosure of this Spiralock.TM. catalog is hereby incorporated by reference and relied upon. Specifically, the thread form produced by the Spiralock.TM. tap incorporates a unique 30.degree. wedge ramp which, at the point clamp loading is applied, contacts the crest of the corresponding male thread creating a continuous spiral contact along the entire length of the thread engagement. This has the effect of spreading the clamp force evenly along all of the threads, thereby reducing the undesirable occurrence of galling and premature component failure.
However, a Spiralock.TM. thread form is not used in a typical jack screw, i.e., power screw, type arrangements since clamping loads necessary to activate the wedge ramp are not consistently encountered. Particularly in prosthetic implants, such as Moore et al., the intermittent loading encountered during normal human activities, such as walking, would yield the Spiralock.TM..
The present invention provides an improvement over the implant disclosed in the Moore et al. patent by increasing the strength of the threaded components of the device while simultaneously decreasing the problem of galling.